Facile Preparation of Functional Group Gradient Surfaces by Desorption and <i>Re</i> -Adsorption of Alkanethiols on Gold

摘要： Many organisms live in various stimuli-responsive milieus of signaling components in gradient forms which are known to be closely related to biological phenomena including embryonic development, wound repair, and tumor metastasis. In addition, the physicochemical properties of gradient surfaces, i.e., a continuous spatial variation of gradient components, enable various types of biological/biochemical research such as cell proliferation/migration and tissue engineering. Various techniques have been reported for the preparation of gradient surfaces, including laminar flow mixing in microfluidic channels, cross-diffusion of alkanethiols, microfluidic permeation printing, oblique deposition and template coating, controlled polymerization on substrates, and controlled UV irradiation on photo-responsive substrates. Previously, we reported a simple method for the generation of multi-component gradient surfaces on self-assembled monolayers (SAMs) on gold. The monolayers consisted of quinone derivatives which reacted with reducing agents in a predictable manner to produce amine gradient surfaces. Recently, desorption and re-adsorption-based methods on SAMs have gained attention, in which alkanethiolates on gold surfaces were desorbed in order to provide an empty space gradient, following which other types of alkanethiols filled up the empty space, leading to an end group gradient of alkanethiolates. For example, Kim et al. reported a simple and flexible method for preparing cell adhesion ligand patterns on SAMs by using a laser beam equipped in a matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer. Similarly, Meyyappan et al. demonstrated that a surface gradient can be achieved by irradiating a focused laser beam on SAMs with subsequent re-adsorption of a second alkanethiol. Fioravanti et al. reported on a new method to fabricate surface chemical gradients of alkanethiols through combining reductive electrochemical desorption and partial re-adsorption of alkanethiolates. Recently, we found that tetrahydrofuran (THF) highly compromises the stability of SAMs on gold, and thus induces unusually fast exchange of alkanethiolates of SAMs with other alkanethiols in the solution. In this study, we harnessed this desorption capability of THF for the preparation of functional group gradient surfaces. In our strategy, a monolayer of alkanethiolates was exposed to THF in a time-dependent manner followed by immersion in a solution of other functional group-terminated alkanethiols, leading to functional group gradient surfaces. The functional group then played a role of a chemical handle to conjugate various functional molecules for the formation of gradients of those components.